This invention relates to a heat-sensitive recording material and particularly to a black color developing heat-sensitive recording material prevented from fading.
There has heretofore been known a heat-sensitive recording material which utilizes color reaction between a colorless or light-colored chromogenic material and an organic or inorganic acceptor, which the two components being thermally brought into contact with each other to produce a developed color image.
One of the essential requirements which such heat-sensitive recording material should meet is that the resulting record image be maintained for a long time without fading under the influence of external conditions, such as light, humidity and heat. The fading of the recording material will bring about a serious hindrance to the usefulness of such recording material. As for improvements in such respect, various investigations and proposals have been made since the development of heat-sensitive recording materials, but each improvement has been attended with new drawbacks, so that satisfactory results have not always been obtained. For example, Japan Kokai (Laid-Open Patent Publication) No. 149,353 of 1975 proposed a method in which a particular amine is added to the color developing layer in order to prevent the fading of the record image due to light. With this method, however, though improvements are observed from the standpoint of fading due to light, the fading tendency of the record image due to humidity and heat is rather increased, sometimes causing the complete loss of the record.
Further, Japan Patent Publication No. 1436 of 1976 proposes a method in which a phenol compound and a phenol resin are jointly used in order to prevent the fading of the record image. With this method, however, not only is the fading-preventive effect insufficient, but also unnecessary coloration, or the so-called "fogging", occurs during the manufacture or storage of the recording material. Since it occurs noticeably under the influence of high humidity and heat, the recording material, if placed under such conditions, would even lose its marketability.
The principal object of the invention is to provide a heat-sensitive recording material which is excellent in the long-term retention of a record image, free from the fading of the record image due to humidity and heat, free from fogging and substantially of uniform quality. Particularly, it provides a recording material aimed to prevent the fading of a record image, especially in the case of a black color developing type heat-sensitive recording material to which the fading of a record image is a vital drawback from the standpoint of practicability.
The above object can be achieved by a combined use of at least two special black color developing fluoran compounds having similar skeletons with an organic acceptor.
Heretofore, in these heat-sensitive recording materials, the mixed use of chromogenic materials having the same developed color hue has not been known or attempted. Still less has it been possible to anticipate that the mixed use, in a particular ratio, of special chromogenic materials having the same blackish hue would contribute to the retention stability of a record image.